Check existence of mysql_version.h.

On RHEL6.4 or similars mysql_config is included in mysql package,
not in mysql-devel. So it's not enough only checking mysql_config,
we should also check header files.

Although it will be nice to replace checking mysql with AX_LIB_MYSQL
or something, I don't do it in this commit because it's out of scope
of this issue.
Use AC_TRY_COMPILE to detect mysql_version.h

Sorry, using "mysql_config --variable=pkgincludedir" to detect
mysql_version.h isn't suitable because "--variable" option doesn't
exist in default package of CentOS6/ScientificLinux6.
I used MySQL-5.5 to test previous commit.
Merge pull request #12 from ashie/check-mysql-version-h

Check existence of mysql_version.h.
fixed the problem that losts trigger data.

Commit: c1515706 introduced the way that partially adds the recently
obtained trigger data. However, the data is added by
DBClientAsura::setTriggerInfoList() that deletes all data before
the new data is saved.
fixed the problem that doesn't save the triggers whose status is OK.
added a test for DBClientZabbix::getItemVariable().
added a member of EventType.
replaced the prefix of EventType members with EVENT_TYPE to match the type name and members.
updated the API reference to reflect the change in commit: 6464a96b..f90ba305.